When inspecting motorcycle hydraulic disc brake pads during a pre-ride check, what is the critical minimum friction lining thickness before pads must be replaced?

1. When the steel backing plate is ground completely flat against the brake rotor
2. When the friction pad thickness reaches 15 mm
3. Brake pads only require replacement when the hydraulic fluid reservoir is completely empty
4. When the friction material wears down to approximately 1.0 to 1.5 mm (or to the depth of the wear indicator groove)